A small, self-contained chamber in which a person could endure months of enforced sleep.

Sometimes Bowman, as First Captain of Discovery, envied his three unconscious colleagues in the frozen peace of the Hibernaculum. They were free from all boredom and all responsibility; until they reached Saturn, the external world did not exist.

But that world was watching them, through their bio-sensor displays. Tucked inconspicuously away among the massed instrumentation of the Control Deck were five small panels marked Hunter, Whitehead, Kaminski, Poole, Bowman. The last two were blank and lifeless; their time would not come until a year from now. The others bore constellations of tiny green lights, announcing that everything was well; and on each was a small display screen across which sets of glowing lines traced the leisurely rhythms that indicated pulse, respiration, and brain activity.

Most fascinating of all were the EEG displays - the electronic signatures of three personalities that had once existed, and would one day exist again. They were almost free from the spikes and valleys, the electrical explosions that marked the activity of the waking brain - or even of the brain in normal sleep. If there was any wisp of consciousness remaining, it was beyond the reach of instruments, and of memory.

This last fact Bowman knew from personal experience. Before he was chosen for this mission, his reactions to hibernation had been tested. He was not sure whether he had lost a week of his life - or whether he had postponed his eventual death by the same amount of time.

When the electrodes had been attached to his forehead, and the sleep-generator had started to pulse, he had seen a brief display of kaleidoscopic patterns and drifting stars. Then they had faded, and darkness had engulfed him. He had never felt the injections, still less the first touch of cold as his body temperature was reduced to only a few degrees above freezing.

Webster's dictionary defines a hibernaculum as "A winter bud, in which the rudimentary foliage or flower, as of most trees and shrubs in the temperate zone, is protected by closely overlapping scales."